drm: Fix lack of CRTC disable for drm_crtc_helper_set_config(.fb=NULL)

Disabling the CRTC by setting its framebuffer to NULL, as used by
drm_framebuffer_cleanup(), was failing to pass the current framebuffer
to the crtc_func->disable callback. This is because of the dance within
drm_crtc_helper_set_config to pass the new_fb (NULL in this case) to the
drm_crtc_helper_set_mode with the currently attached fb as a parameter.
drm_crtc_helper_set_mode treats this as a no-op and the encoder is still
enabled. And so the current fb is forgotten before the call to
drm_helper_disable_unused_functions.

This patch treats disabling the CRTC as a simple special case rather
than adding further complexity into the configuration logic.

This fixes a pin-leak of the fb bo on Xserver close.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_crtc_helper.c

@@ -456,6 +456,30 @@ bool drm_crtc_helper_set_mode(struct drm_crtc *crtc,
 EXPORT_SYMBOL(drm_crtc_helper_set_mode);
 
 
+static int
+drm_crtc_helper_disable(struct drm_crtc *crtc)
+{
+	struct drm_device *dev = crtc->dev;
+	struct drm_connector *connector;
+	struct drm_encoder *encoder;
+
+	/* Decouple all encoders and their attached connectors from this crtc */
+	list_for_each_entry(encoder, &dev->mode_config.encoder_list, head) {
+		if (encoder->crtc != crtc)
+			continue;
+
+		list_for_each_entry(connector, &dev->mode_config.connector_list, head) {
+			if (connector->encoder != encoder)
+				continue;
+
+			connector->encoder = NULL;
+		}
+	}
+
+	drm_helper_disable_unused_functions(dev);
+	return 0;
+}
+
 /**
  * drm_crtc_helper_set_config - set a new config from userspace
  * @crtc: CRTC to setup
@@ -510,8 +534,7 @@ int drm_crtc_helper_set_config(struct drm_mode_set *set)
 				(int)set->num_connectors, set->x, set->y);
 	} else {
 		DRM_DEBUG_KMS("[CRTC:%d] [NOFB]\n", set->crtc->base.id);
-		set->mode = NULL;
-		set->num_connectors = 0;
+		return drm_crtc_helper_disable(set->crtc);
 	}
 
 	dev = set->crtc->dev;